Actually, few prisoners were executed within the Tower of London. Most were carried out publicly at various London sites, including on nearby Tower Hill. Only the most high profile or sensitive cases involving royalty or nobility were afforded a more private execution inside its walls.
A memorial was originally ordered by Queen Victoria in 1866, but has been replaced by this new memorial, unveiled in September 2006. It is intended to remember all those executed there over the years, although the upper glass circle bears just ten engraved names, seven of the most famous beheaded which include Anne Boleyn, Catherine Howard, Lady Jane Grey and the Earl of Essex, and those of three unfortunate Black Watch soldiers. They and about 100 colleagues had gone absent from duty when they were all recalled to London whilst en route for leave in Scotland in 1743. Charged with mutiny, all were eventually pardoned except these three who were shot at the Tower by a firing squad of their colleagues. The memorial is topped by a glass-sculpted pillow at its centre, and the lower circle of dark stone bears a poem around its rim.